The invention is directed to a method for nondestructive testing of pipes for surface flaws.
Nondestructive methods for testing metal pipes for surface flaws, for example by ultrasound testing, are known for some time and have proven to be reliable.
Ultrasound tests are used in production to check, in particular, that the required wall thickness of the pipe is maintained and to detect potential discontinuities disposed in the pipe wall, for example laminations, cracks, grooves, scrap marks and other surface flaws.
With the pulse echo method, ultrasound pulses are excited in the wall during a test, starting from the exterior surface of the pipe, and the signals reflected by the interior surface of the pipe are received. The thickness of the pipe wall can be calculated from the transit time of the signal and the speed of sound in the tested material. This method is typically employed automatically during production for both magnetizable and non-magnetizable pipe materials.
This method has the disadvantage that, in particular, flaws on the interior side of the pipe, such as a bulges, which may very gradually taper on and off, could only be detected with great difficulty or not at all when employing the evaluation methods currently used in ultrasound testing.
For flaws having a curved surface, the ultrasound signals are reflected in different directions by scattering. The test head then receives the reflected ultrasound signals either not at all or not completely, so that a signal originating from a flaw is no longer unambiguously distinct from the inherent noise level of the signals and can therefore no longer be detected.
The method for detecting bulges disposed on the interior wall of the pipe, as disclosed in DE 100 65 093 A1, is also does not provide guidance. The method described therein is based on evaluating the reflected ultrasound signals to determine the magnitude by which the signal strength of the echo pulses (sequence of echoes from the rear wall) decreases. However, a bulge can still not be unambiguously identified by this method, because the decrease in the signal strength of the echo pulses may also have other causes, for example non-critical interior flaws or geometric effects.
It would therefore be desirable to investigate filter techniques suitable for separating flaw-based signals from the inherent noise level. In addition to digital filtering with conventional filtering algorithms, the so-called wavelet algorithms are particularly suited for this task. Instead of harmonic functions, wavelets are used as filter criteria because these can be very similar to the useful signals. By using wavelet filters, noise can be much more effectively reduced than with conventional filtering techniques.
It is generally known, for example from DE 102 25 344 A1, to use a wavelet transformation for evaluating time-dependent signals in industrial process monitoring to separate the noise components of the signals from the information components of the signals. In a wavelet transformation, which is an extension of the Fourier transformation, the original signal is projected onto wavelet basic functions, which corresponds to a transformation from the time domain to the time-frequency plane. The wavelet functions which are localized in the time domain and in the frequency domain, are derived from a single prototype wavelet, the so-called mother function, by dilatation and translation.
The intent is here to significantly reduce with the wavelet transformation the noise level compared to the signal caused by the flaw.
The conventional method discloses in general terms the advantages of applying of the wavelet algorithm to noise suppression for monitoring industrial processes. It is imperative with pipes produced in a continuous production process that the signals from the nondestructive testing are analyzed in near-real-time, so as to be able to immediately change the production process when flaws occur (for example, correlating the flaw by marking the pipe section or stopping the production process). However, DE 102 25 344 A1 does not address this issue.
WO 2005/012941 discloses a method for nondestructive testing of objects using ultrasound waves, wherein a wavelet transformation is used to reduce and/or compress the amount of data. Noise reduction or signal separation are not performed.
Therefore, there remains a problem during ultrasound testing in that surface test data of pipes must be measured and processed in near-real-time so as to allow intervention in the ongoing production process when flaws occur.